This invention relates to a collector and, more particularly, to a multistage depressed collector of improved efficiency used to collect moving electrons. Many electronic devices employ a traveling stream of charged particles, such as electrons, formed into a beam as an essential function in the device's operation. For example, one type of microwave tube, the traveling wave tube, incorporates a source of electrons that are formed into a beam, in which the electrons are accelerated to a predetermined velocity and directed along an axial path through an "interaction" region within the microwave tube body. In the interaction region, kinetic energy is transferred from the moving electrons to the high frequency electromagnetic fields, such as microwave signals, that are propagating along a slow wave structure through the interaction region at about the same velocity as the moving electrons. The electrons give up energy to the microwave field through the exchange process characterized as electronic interaction, evidenced by a lower velocity of the electrons exiting from the interaction region. The "spent" electrons pass out the interaction region where they are incident upon and collected by a final tube element, termed the collector. The collector collects and returns the incident electrons to the voltage source. As is recognized, much of the energy in a moving particle is released in the form of heat when the particle strikes a stationary element, such as the collector. This produces undesired heating in the microwave tube and a lower overall electrical efficiency of microwave tube operation.
The depressed collector as is known and, more particularly, the multistage depressed collector is a collector that increases the electrical efficiency of traveling wave tube operation as well as reduces undesirable heat generation by a process of velocity sorting of the electrons controlled by a retarding electric field. The field slows the electrons so that the electrons are collected by the electrodes at a reduced velocity and ideally at a zero velocity. As is known to those skilled in the art, the multistage depressed collector is characterized physically by a series of spaced metal electrodes, each containing a passage therethrough, a final electrode and a passage entry for receiving electrons. The electrodes are maintained at successively lower voltages with respect to the tube circuit taken as ground (or at successively higher negative voltages as otherwise viewed) so as to present a retarding electric field to the electrons which pass through the entrance into the collector region. Such types of devices are substantially well developed and hence are complex in nature as is known to the reader skilled in the art.
As far as known from the literature, the two most efficient structures in multistage depressed collectors prior to this invention are the so-called Japanese collector and the NASA-GE collector. By way of background, the reader may make reference to the following patents: U.S. Pat. No. 3,526,805 to Okoshi et al; U.S. Pat. No. 3,644,778 to Mihran et al; and U.S. Pat. No. 3,702,951 to Kosmahl, and to the following publications: IEEE Transactions on Electron Devices, Vol. ED-19, No. 1, Jan. 1972, pp 104--110; The Titled Electric Field Soft Landing Collector and Its Application to a Traveling Wave Tube, Okoshi et al; IEEE Transactions on Electron Devices, Vol. ED-19, No. 1, Jan. 1972, pp 111-121; A Ten-Stage Electrostatic Depressed Collector for Improving Klystron Efficiency, Neugebauer et al; and Multistage Depressed Collector Investigation for Traveling Wave Tubes, Tammaru, NASA CR-72950 EDDW-3207, Contract NAS 3-11536 Final Contract Report, should the reader desire to become acquainted with the structure of the foregoing in somewhat greater detail than is here presented.
The Japanese collector employs a combination of transverse electric field and a longitudinal magnetic field for sorting electrons as a function of the electron velocity. The NASA collector employs a retarding electric field established by a cuplike electrode and a pointed spike located in the center of the cuplike member. The effect of such structure with a voltage applied is to present an electron mirror with a negative focal length to electrons moving near the axis. Hence the reflected beam is more divergent than the incident beam. The efficiency of the aforementioned NASA collector is limited by the defocusing properties of the spikelike reflector element. In addition, some electrons may strike the spikelike element which, in turn, generates secondary electron emission and these secondary electrons may be accelerated back into the interaction region of the tube to cause difficulty. The Japanese collector as was noted, requires the maintenance of an axial magnetic field of a critical magnitude for proper functioning. As a result, the collector is not suited for high power operation.